X-Motor Racing - Real 3D Racing Simulator

Theory

From blog

When a vehicle drives it contact with a road by tires so tires is the most important thing. It's logically start from tires and some theory. At the picture below you can see Tire Axis System that XMR uses.

A vehicle accelerates and brakes because of applied longitudinal tire force (Z axis). If Z > 0 then tire accelerate a vehicle. If Z < 0 then tire brakes a vehicle. A vehicle turns because of applied Lateral tire force (X axis). If X > 0 then tire turns a vehicle to the right, else if X < 0 then tire turns a vehicle to the left. A tire provides longitudinal and lateral forces if Normal force (Y) > 0. If Normal force is equal 0 then X and Z must be 0 also.
You can see tire forces on this video.

Longitudinal and lateral forces depend on slip ratio (slip) and slip angle. Camber thrust adds lateral force if camber angle is not zero. Longitudinal force depends on slip ratio. XMR uses Sakai slip definition:
Traction Slip = V * cos (A) / w*R - 1
Braking Slip = 1 - (w*R)/(V*cos(A))
V - speed of the tire over the roadway
A - slip angle of the tire.
w - angular velocity of the tire
R - effective radius of the tire
Slip angle is an angle between a rolling tire's direction and the direction of the tire heading. You can see that Traction Slip < 0 and Braking Slip > 0. Slip is limited by [-1..1] For example, traction slip is -0.5 when the tire spinning (2X free rolling). -1 Slip when tire spinning (infinite). At this picture you can see a typical longitudinal forces (tractive or braking) diagram when a slip angle is zero and tire load is equal to 4 kN.

You can see when the tire has the maximum tractive and braking forces. Tractive force at -1 Slip is less than Tractive force at -0.5 Slip. The same behavior for braking forces. On the picture below you can see a typical lateral force diagram when a slip is zero and tire load is equal to 4 kN.

You can see that lateral force is max at 7 degree of the slip angle. Lateral force is decreasing with the increasing of the slip angle. So you will have less grip at large slip angle. And a short video to see slip and slip angle

Vehicle Physics
VehiclePhysics is a powerful tool to tune vehicle physics. You can tune the most of the parameters real-time, just launch the VehiclePhysics from the game. It will open the physics profile. To tune physics, it's more convenient to launch the game in the window mode.
VehiclePhysics is divided into two parts. The left part contains the list of the cars, physics profiles and some options. The right part contains some dialogs/tab for the physics options. Select the car you like to tune then select the physics profile from the PHYSICS PROFILE. Make any changes and then press the UPDATE to save them. If the game is running with the physics profile then all the changes will be applied at once. You just need to test them.

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